Japanese Laid-Open Patent Publication Nos. 2002-227688 and 2003-020930 describe a technique that is used when a large amount of particulate matter (PM) accumulates in a filter, which is arranged in an exhaust system of a diesel engine. In such a case, the air-fuel ratio of the exhaust is repeatedly adjusted between a rich state and a lean state to heat the filter and burn the particulate matter in the filter.
However, if the air-fuel ratio of the exhaust is repeatedly adjusted between a rich state and a lean state when a large amount of particulate matter is accumulated in the filter, a large amount of particulate matter would be suddenly burned. This may cause overheating of the filter and cause thermal deterioration.
To prevent overheating when burning a large amount of particulate matter, the filter may be heated when the amount of particulate matter accumulated in the filter is relatively small so as to burn the particulate matter in small amounts. However, this would increase the frequency of filter heating. Thus, the consumption of fuel used to enrich the air-fuel ratio would increase and thus reduce fuel efficiency.
In another technique, particulate matter is slowly burned when the air-fuel ratio of exhaust is relatively low (e.g., when the air-fuel ratio is slightly greater than the stoichiometric air-fuel ratio). When the state of the engine is changing or when the accumulation of the particulate matter is uneven, there may be errors in a detection value or estimation value representing the amount of accumulated particulate matter. Accordingly, the heating of the filter may end when the particulate matter is still not completely eliminated from the filter. Such residual particulate matter may cause sudden burning of a large amount of particulate matter and thus overheat the filter.